High performance thermoplastic polymers such as poly(etherimide)s have been used to fabricate parts for numerous applications. Each application requires particular tensile and flexural properties, impact strength, heat distortion temperature (HDT), and resistance to warp. For example, U.S. Pat. No. 4,455,410 provides a poly(etherimide)-poly(phenylene sulfide) blend having good flexural strength characteristics. U.S. Pat. No. 3,983,093 provides poly(etherimide) compositions that have improved solvent resistance and are suitable for use in preparing films, molding compounds, coatings, and the like.
These thermoplastic polymers are characterized by a high glass transition temperature, usually above about 180° C., which makes them suitable for use in applications that require exposure to high temperatures. A drawback of these materials is that they exhibit poor melt flow properties, which makes processing difficult. Injection molding of thermoplastic polymers, for instance, is more easily performed with a thermoplastic resin that has a higher melt volume rate (MVR). Good melt flow properties are necessary to achieve fast molding cycles and to permit molding of complex parts. At the same time, mechanical properties such as impact strength and ductility must be maintained in order to pass product specifications.
U.S. Pat. No. 4,431,779 to White et al. discloses blends of polyetherimide and polyphenylene ether which exhibit good impact strength as well as good mechanical properties. White et al. focus on the compatability of polyphenylene ethers with polyetherimide, teaching that homogenous blends and non-uniform products may result. However, if amorphous polymers are employed, they are compatible and transparent films may be cast. The compatibility of the polyphenylene ethers to polyetherimide lessens as the quantity of aliphatic groups in the polymer increases. Although White et al. discuss polypheneylene ether polymers, they fail to teach the effects of such polymers and polymer blends on melt flow characteristics.
There accordingly remains a need in the art for thermoplastic polymers with improved melt flow properties, without the consequent loss of other desirable characteristics in the finished product.